1. Field of the Invention
This invention relates to a process for preparing acetic acid using carbon monoxide gas and hydrogen gas as raw materials. Carbon monoxide gas and hydrogen gas are catalytically reacted under elevated pressures in a liquid medium containing a ruthenium compound, cobalt compound and promoter(s).
2. Description of the Prior Art
There have been known several examples of the so-called direct process which uses carbon monoxide gas and hydrogen gas as raw materials to directly synthesize acetic acid. For example, U.S. Pat. No. 4,235,801 and U.S. Pat. No. 4,246,186 disclose a process for obtaining a hydrocarbon and an oxygen-containing compound by reacting carbon monoxide and hydrogen gas in the presence of rhodium carried on a carrier such as silica gel in accordance with the gas-phase heterogeneous process. According to the above process, the selectivity of carbon monoxide consumed during the reaction into acetic acid (hereinafter called "acetic acid selectivity") was however limited to low values of 30% and so. Furthermore, the above process is accompanied by such a drawback that hydrocarbons such as methane and the like occur as by-products in large volumes and their separation from the gaseous mixture of unreacted carbon monoxide and hydrogen gases is extremely difficult. On the other hand, U.S. patent application Ser. No. 590,717 filed June 26, 1975 and U.S. Pat. No. 4,014,913 disclose a similar gas-phase heterogeneous process which makes use of a rhodium-manganese catalyst instead of the above-described rhodium. However, the acetic acid selectivity of this process is still around 30% or so. It is difficult to say that the by-production problem of hydrocarbons has been improved. Furthermore, U.S. Pat. Nos. 4,224,236 and 4,288,558 (DE-OS No. 2,814,427 and DE-OS No. 2,814,365) describe another gas-phase heterogeneous process in which the reaction is carried out in the presence of a catalytic system of rhodium added with magnesium and a base. According to this process, the acetic acid selectivity has been improved to a substantial extent. However, it still produces the by-product, methane, as much as several percents to forty percents.
There is thus a strong demand for the development of a preparation process of acetic acid, which process has improved the drawbacks of such prior art processes as referred to in the above.